Benefits of the Real-time Approach Peter Bownes HDR Prostate - - PowerPoint PPT Presentation

Benefits of the Real-time Approach

Peter Bownes

HDR Prostate Workshop Nov 2013

Real Time US Pathway

US Based Pathway

CT Based Planning Pathway

Treatment Accuracy

3D Imaging Target Localisation Target Definition Insertion

• Applicator

fixation

• Guidance

Applicator Reconstruction Dose Calculation Biological Model HDR Accuracy

• time/position

“A chain is no stronger than its weakest link”

Contouring

Needle Reconstruction Plan Adaption Needle Displacement

Imaging Modalities

MR Image courtesy of G Lowe, Mount Vernon Hospital

T2 Weighted MRI US - BK Flex Focus

Imaging Modalities

MR & CT images courtesy of G Lowe, Mount Vernon Hospital

T2 Weighted MRI BK Flex Focus CT 3mm Slice Thickness

Image Acquisition for Contouring

Modality Slice Thickness Advantages Disadvantages

MRI T2 Weighted Typically 2 – 3mm

• “Gold Standard” for prostate

definition – good soft tissue defn

• Good for OAR definition

(snap shot”)

• Easier to fuse mpMRI
• Non ionising
• Patient transfers – Legs down
• Time delay
• Resolution – SNR/Time balance
• Additional scan/fusion of needle

recon

• Not real-time

US Transverse Or US Sagittal 1mm 0.5° rotation ECRM

• Excellent resolution
• Good definition of prosatate

margin/bladder/sem ves

• Real time feedback – aid base

and urethra definition.

• Information from virtual stage
• Needle position as surrogate
• Non-ionising
• Image quality deteriorates after

needle insertion (artefact – needle shadowing / false catheter tracks)

• Quality dependent on good

acoustic coupling CT Typically 2 – 3mm

• Good for needle recon
• Adequate for OAR

(“snap shot”)

• Inferior soft tissue definition
• Patient transfers – Legs down
• Time delay
• Resolution – dose / noise balance
• Patient dose
• Not real-time

Prostate Volume Comparison

• McLaughlin IJROBP 54 (3) 703-711, 2002

– 45 I-125 patients – T2 weighted MR “gold standard” – superior at:

• Pelvic diaphragm
• Apex v soft tissue
• Base v SV
• Base v Bladder

CTpost/MRpost CTpost/USpre MRpost/USpre MRpost/MRpre 1.34 (SD 0.35) 1.40 (SD 0.35) 1.07 (SD 0.26) 1.10 (SD 0.2)

Aid to target definition (Virtual – Live)

3.5cm 3.5cm 4.5cm 36.5cc Base Apex Needle

OAR stability

• CT/MR “snap shot” for

planning

• Rectal shape/size/position

can change

• US probe at base

– Rectal position stable – Real time verification – Time between plan/treat reduced

Contouring

Needle Reconstruction

Plan Adaption Needle Displacement

Image Acquisition for needle reconstruction

Modality Slice Thickness Advantages Disadvantages

US Transverse US Sagittal 1mm 0.5° rotation ECRM

• Fine resolution
• Real time confirmation of tip
• Free length confirms tip

position

• Non-ionising
• Automatic
• Tip difficult to define with US alone
• Needle artefact (shadowing)
• False catheter tracks (blood left

when catheter retracted) CT Typically 2 – 3mm

• Visualisation of catheter/tip
• Patient transfers – Legs down
• Time delay
• Partial Volume effect – Tip location
• Patient dose
• Resolution – dose / noise balance
• Not real-time

MRI Typically 2 – 3mm

• 3D sequences available
• Non ionising
• Poorer visualisation to CT
• Patient transfers – Legs down
• Time delay
• Resolution – SNR/Time balance
• Distortion/patient movement

US Catheter Reconstruction

• Requires good probe contact with

rectal wall

• Main uncertainty on ultrasound is the

location of tip

– Needle artefacts – Blood artefacts

• How is uncertainty minimised to

acceptable levels

– Catheter “Free-Length” – Real time guidance

• Zheng et al* Average tip-detection

accuracy 0.7mm [max 0.8mm] (compared to X-ray)

Mark on Probe Cradle locking device for probe

Free Length

* Brachytherapy 10 (2011) 466-473

Inter-observer variation in applicator reconstruction on OCP

MSc Project L Partridge, Uni of Leeds 2008

• 2 Cases
• 6 Observers
• Free Length provided
• Observer 1 –

reference standard

90% 91% 92% 93% 94% 95% 96% 97% 98% 99% 100%

• 3.5
• 3
• 2.5
• 2
• 1.5
• 1
• 0.5

0.5 1 1.5 2 2.5 3 3.5 V100 prostate Offset (mm)

1 2 3 4 5 6 7

Cranial Caudal

How reconstruction error effects V100prostate

± 0.7mm equates to <0.5% V100pros

Are there issues with CT?

• Partial volume effects
• Effect Tip detection
• Random errors depend on slice

thickness

• Slice thickness – dose/noise balance
• Window settings
• Effects interpretation of tip location
• Hypo-intense regions

CT Slice

Contouring Needle Reconstruction

Plan Adaption

Needle Displacement

• Real Time Approach
• Allows for additional needle insertion if cold spots are

seen in the dosimetry

• Real Time Approach
• Allows for needle position adaption
• Difficulty in achieving planning aims (target coverage & OAR

constraints)

• Needle placement is the most critical factor in

achieving good dosimetry

• Knowledge of achieved dosimetry while in theatre

Contouring Needle Reconstruction Plan Adaption

Needle Displacement

Needle Movement

Planning to Treatment Delivery

Results Mean Interfraction movement of catheters relative to the prostate (CTplan – CT2nd) = 7.9mm; (range 0-21mm) (CTplan – CT3rd) = 3.9mm; (range 0-25.5mm)

• 20 Consecutive monotherapy

patients

• 326 catheters
• 3 Fractions of 10.5Gy (30-36 hrs)

Mean Catheter Displacement – CT Based

Simnor et al Radiother. Oncol 93, 253-258 (2009)

Implant CTplan 1st # 2nd # CTQA 3rd # CTQA

0 1-2 4-6 24 30 (Hrs)

Simnor et al Radiother. Oncol 93, 253-258 (2009)

• 25 patients - monotherapy
• 3 Fractions of 11.5Gy
• 2wks between # - separate implant
• Probe @base during treatment
• Prostate Vol mean 35.8 cm3

(17.4 cm3 to 59.6 cm3)

• Analysed volume changes, needle

displacement and dosimetric impact

Mean Catheter Displacement – Real-time US

Milickovic et al Med. Phys 38(9), 4982-4993 (2011)

Implant USplan Treatment USpre-irrad

Mean Time (mins)

USpost-irrad Study

51.2 (37–65) 19.3 (16-24)

Results Prostate – rigid shift mean 0.57mm (0-2.1mm) Urethra - largest shift @ base mean (plan to post) 1.1mm (0-5.1mm) Rectum mean (plan to post) 0.4mm (0-1.4mm) Needle displacement mean 1mm (<1.5mm, except for one)

Milickovic et al Med. Phys 38(9), 4982-4993 (2011)

Mean Catheter Displacement Against Time

Simnor et al Radiother. Oncol 93, 253-258 (2009)

Simnor (2009) 20 mono 3 Fractions 30-36h CT Scans (3mm) Tip @ 1st # Yes Simnor Milickovic Milickovic (2011) 25 mono 3 Fractions US Tip/Free length Yes

Conclusion (1)

• US Contouring
• Excellent resolution of US acquisition
• Real time aids definition
• Virtual information can help with needle artefacts
• Better soft tissue definition to CT
• Needle reconstruction
• Fine resolution of US acquisition, real time tracking and free-length measurements

allow accurate reconstruction

• Plan Adaption
• Additional needles / adjustment of needle position can be made after first iteration of

planning.

• Dosimetry known whilst still in theatre

Conclusion (2)

• Needle Displacement – Planning to Treatment
• Time of procedure (implant to treat) significantly reduced (less than 2hours)
• Reduces the level of oedema at time of treatment
• Reduces catheter displacement (<1mm) – minimal dosimetric impact
• Treatment delivery position = Planning position
• Reduces movement of patient (avoids: theatre-recovery-ward-CT-ward-treatment)
• OAR position stable (US probe at base)
• Patient Comfort – entire treatment completed under GA; no corrective

action for needle displacement

• Team in same place
• Uncertainties minimised if separate insertion/plan for each fraction

Thank You

peter.bownes@leedsth.nhs.uk